This invention concerns radomes. Examples of radomes are disclosed in U.S. Pat. Nos. 4,615,933, 4,615,859, 4,522,883, 4,506,269, 4,364,884, 3,680,130, 3,496,604, 3,396,220, 3,293,544 and 3,095,162. This invention is particularly concerned with fused silica radomes such as are disclosed in U.S. Pat. Nos. 3,680,130 and 3,336,873.
Fused silica is a material whose thermal shock resistance, low dielectric constant, low loss tangent, and high temperature capability make it an excellent candidate for missile radomes.
In the slip casting process, carefully controlled particle size distributions of fused silica powder are suspended in water and cast into plaster of paris molds. After careful drying in controlled humidity conditions the parts are then "sintered" to final density.
The difficulty in sintering fused silica is that densification occurs by a viscous flow mechanism and that the temperature required to achieve substantial densification also causes crystallization. The crystallization reduces the thermal shock capabilities of the material.
The current practice is therefore to control the powder particle size distribution during casting such that as-cast densities of over 80% of the theoretical density value are obtained followed by a sintering which only increases density to about 85% of full density.
The drawbacks of current fused silica radome material lie primarily with the low density, which in turn leads to low strength, low rain erosion resistance, and the need to seal the radome against moisture.